1. Field of the Invention
The present invention is directed to a method for slice position planning of tomographic (magnetic resonance) measurements, including a protocol for operating a magnetic resonance imaging apparatus.
2. Description of the Prior Art
Magnetic resonance imaging (MRI, also known as magnetic resonance tomography (MRT)) is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized as an imaging modality in medicine and in biophysics for more than fifteen years. In this imaging modality, a subject, such as a living patient, is subjected to a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject, that were previously irregularly oriented, are aligned. Radio-frequency energy emitted into the subject then excites these “ordered” nuclear spins to a specific resonance. This resonance generates the actual measurement signal, which is received with suitable reception coils. By utilizing non-uniform magnetic fields (gradient fields) generated by gradient coils, the signals received from the examination subject can be spatially encoded in all three spatial directions. A slice of the examination subject for which an image is to be generated can be freely selected, thereby allowing tomograms of the human body to be obtained in all orientations. Magnetic resonance imaging as a tomographic method for medical diagnostic purposes is primarily distinguished as a “non-invasive” examination technique with versatile contrast capability. Due to the excellent presentation of soft tissue, magnetic resonance imaging has developed into an imaging modality that is often superior to x-ray computed tomography (CT). Magnetic resonance imaging is currently based on the use of spin echo sequences and gradient echo sequences that enable an excellent image quality to be obtained, with measurement times on the order of magnitude of minutes.
Each examination (scan) of a subject in a particular magnetic resonance imaging installation must be planned in advance. The planning involves selection of the type of pulse sequence, as well as the selection or designation of many individual parameters of the selected pulse sequence. The selection of the pulse sequence and the parameterization thereof are, in turn, based on many variables that differ from scan-to-scan. Such variables are related to the specific patient, the type of imaging installation, and the particular type and orientation of the magnetic resonance image that is desired to be obtained. The image to be obtained is dependent not only on anatomical factors, but also on the particular pathological condition, or suspected pathological condition, that is being investigated.
For clinical MR scanners, protocols are predefined with regard to slice positioning, but such protocols are not based on the actual positioning of the patient in the scanner for the particular examination to be undertaken. Usually, the protocols are defined relative to the center of the origin of the basic field magnet, which usually also is the origin of the imaging volume, and straightforward axial, sagittal or coronal slices are selected depending on the preferred protocol orientation. For performing the actual scan, the final slice position must be adjusted manually, otherwise the slice will not coincide with the desired body region of the subject. In principle, this manual procedure must be performed with regard to every protocol and every patient. This not only prolongs the time that the patient must spend in the scanner, which is discomforting to the patient, but also slows the patient throughput (i.e., results in a smaller number of patients being scanned within a given time than would be possible without such manual positioning).
Conventionally, such manual re-alignment of the slices for the actual scan, compared to the slice alignment in the predefined protocol requires the use of a so-called localizer protocol. This involves positioning the patient in the scanner, undertaking a localizer scan, position the slices for the actual diagnostic scan based on the images obtained in the localizer scan, and undertaking the clinical or diagnostic scan from which diagnostic images will be obtained.
In the context of conventional slice position planning, the use of templates is described in U.S. Pat. No. 6,195,409, and the processing of medical images employing techniques suitable for slice position planning is described in PCT Application WO 02/43003. The mapping of a particular property in the context of image processing is disclosed in PCT Application WO 02/098292, and the registration of object views is described in PCT Application WO 01/59708.